In order to imitate the behavior of a tiny organic molecule, a team of quantum computer scientists at UNSW Sydney have developed an atomic-scale quantum processor, meeting the challenge posed almost 60 years ago by theoretical physicist Richard Feynman. The accomplishment, which happened two years ahead of schedule, marks a significant turning point in the quest to create the first quantum computer in history and shows the team’s capacity to exquisitely regulate the quantum states of electrons and atoms in silicon.
The researchers revealed how they were able to simulate the structure and energy states of the organic complex polyacetylene—a repeating chain of carbon and hydrogen atoms differentiated by alternating single and double carbon bonds—in an article that was published today in the journal Nature.
The team at Silicon Quantum Computing, one of UNSW’s most exciting start-ups, built a quantum integrated circuit consisting of a chain of 10 quantum dots to simulate the precise location of atoms in the polyacetylene chain, according to lead researcher and former Australian of the Year Scientia Professor Michelle Simmons. Richard Feynman claimed in the 1950s that it is impossible to comprehend how nature functions unless one is able to construct matter at the same length scale, according to Prof. Simmons.
In order to duplicate the polyacetylene molecule, we are basically creating it from the ground up by placing silicon atoms at precise intervals to stand in for the single and double carbon-carbon bonds.
Chain reversal
The study depended on monitoring the electric current flowing through a purposefully created 10-quantum dot polyacetylene molecule replica as each new electron moved from the device’s source outlet to the drain, or other end of the circuit.
They simulated two separate polymer chain strands to be doubly certain.
In the first device, a portion of the chain was severed, leaving double bonds at the end and producing 10 current peaks. In the second device, a separate section of the chain was severed, resulting in single bonds at the end and just two current peaks. Because the atoms at the chain’s end have varied bond lengths, the current that flows through each chain is consequently drastically different.
The measurements not only agreed with the theoretical predictions, but they agreed exactly.
What it demonstrates is that you can accurately simulate what occurs in a real molecule. The signatures of the two chains are significantly different, which is why it’s fascinating, according to Prof. Simmons.
The majority of existing quantum computer systems lack the capability to manufacture atoms with sub-nanometer accuracy or to allow them to be placed so closely together.
Therefore, by arranging the atoms such that they resemble the actual physical system, we may now begin to grasp increasingly complex compounds.
Positioned near the edge
Prof. Simmons claims that the selection of a carbon chain made up of 10 atoms was not by chance because it falls within the range of computations that a classical computer can do, with a system including up to 1024 different electron interactions. By increasing it to a 20-dot chain, the number of potential interactions would exponentially increase, making the problem challenging for a traditional computer to complete.
It’s like going over the cliff into the unknown, she adds, since we’re getting close to the limit of what conventional computers can achieve.
“The fascinating part is that we can now create larger devices that go beyond what a traditional computer can model. Consequently, we are able to examine never-before-simulated molecules. By tackling basic issues that we have never been able to resolve, we will be able to comprehend the world in a new way.”
Understanding and imitating photosynthesis—the process by which plants use light to produce chemical energy for growth—is one of the issues Prof. Simmons pointed to. Or learning how to make the existing expensive and high-energy process of designing catalysts for fertilizers more efficient.
There are significant ramifications for our fundamental knowledge of how nature functions, she continued.
Atomic clocks in the future
In the past three decades, a lot has been written about quantum computers, but the million-dollar question has always been, “but when can we see one?”
According to Prof. Simmons, the evolution of quantum computers is following a similar path to that of classical computers, which started with a transistor in 1947 and progressed to an integrated circuit in 1958 before small computing chips were introduced into consumer goods like calculators about five years later.
And therefore, according to Prof. Simmons, “we’re repeating that blueprint for quantum computers.”
There are significant ramifications for our fundamental knowledge of how nature functions, she continued.
“In 2012, we began with a transistor made of just one atom. And this most recent achievement, accomplished in 2021, represents a two-year headstart on the atom-scale quantum integrated circuit. In five years, if we map it to the development of classical computing, we should have some sort of commercial use for our technology.”
The method is scalable thanks to the UNSW/SQC team’s research since it manages to employ fewer components in the circuit to control the qubits—the fundamental bits of quantum information.
There are significant ramifications for our fundamental knowledge of how nature functions, she continued.
In order to produce a quantum state in a device, Prof. Simmons explains, “you need something that makes the qubits in quantum systems.”
“Since the qubits in our system are produced by the atoms, fewer components are needed in the circuits. In our 10-dot system, we only need six metallic gates to govern the electrons; hence, we have fewer gates than there are active device components. While the majority of quantum computer designs require two or more times as many control systems to move the electrons, the qubit architecture just requires one.”
There are significant ramifications for our fundamental knowledge of how nature functions, she continued.
The interference with the quantum states is reduced by having fewer components packed closely together, which enables devices to be scaled up to create more intricate and potent quantum systems.
“Because it demonstrates that we have this well-maintained system that we can manage, maintaining coherence over great distances with little gate overhead, the extremely low physical gate density is also quite intriguing for us. It is useful for scalable quantum computing because of this.”
In the future, Prof. Simmons and her team will investigate bigger compounds, including high temperature superconductors, that may have been theoretically predicted but have never been simulated and thoroughly understood.